JPS63169333A - Production of chromium stainless steel strip of double phase structure having small intra-surface anisotropy and high ductility and high strength - Google Patents

Abstract

PURPOSE:To obtain a stainless steel strip of double phase structure having small intra-surface anisotroy and having high ductility and high strength by hot rolling a steel slab having a specific compsn. essentially consisting of Cr, then subjecting the slab to cold rolling including intermediate annealing and holding the rolled sheet in a specific temp. region in a continuous heat treatment furnace then subjecting the sheet to controlled cooling. CONSTITUTION:The slab of the steel consisting of <=0.15wt.% C, <=2.0% Si, <=1.0% Mn, <=0.040% P, <=0.030% S, <=0.60% Ni, 14.0-20.0% Cr, <=0.12% N, <=0.02% O, and the balance Fe and unavoidable impurities and satisfying the relation expressed by the formula is produced. Said slab is then subjected to hot rolling and >=2 passes of cold rolling including intermediate annealing under heating to the single phase region temp. of ferrite to the product sheet thickness. The rolled sheet is passed through the continuous heat treatment furnace where the sheet is held at the two-phase region temp. of ferrite + austenite of Ac1 point or above and <=1,100 deg.C within 10min and is then cooled at 1-500 deg.C/sec average cooling rate from the max. heating temp. down to 100 deg.C. The chromium stainless steel strip of double phase structure having >=200HV hardness, the small intra-surface anisotroy and the high ductility and high strength is obtd. by the above-mentioned finising heat treatment.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a new industrial manufacturing method for a high-strength multi-phase chromium stainless steel strip with excellent ductility and low in-plane anisotropy of strength and ductility. The present invention provides a method for manufacturing a high-strength, high-ductility stainless steel strip that is used as a forming material that requires high strength and is subjected to processing such as press forming.

[Background of this field]

Chromium stainless steel containing chromium as a main alloy component includes martensitic stainless steel and ferritic stainless steel. Both are cheaper than austenitic stainless steels, which contain chromium and nickel as the main alloy components, and have physical properties that are not found in austenitic stainless steels, such as ferromagnetism and a small coefficient of thermal expansion. Therefore, there are many uses that are limited to chromium stainless steel not only for economic reasons but also for characteristics. Particularly in the fields of electronic equipment and precision mechanical parts in recent years, demand for chrome stainless steel sheets has increased, and in applications where chrome stainless steel sheets are used, processing products are becoming more functional, smaller, more integrated, more precise, and the processing process is simplified. requirements are becoming increasingly strict. For this reason, in addition to the corrosion resistance of stainless steel mesh and the above-mentioned characteristics of chrome stainless steel, the material of the chrome stainless steel plate is required to have even higher strength, workability, and precision. Therefore, the emergence of chromium stainless steel sheet materials that have the contradictory characteristics of high strength and high ductility, excellent shape and thickness accuracy at the time of raw steel sheet, and excellent shape accuracy after processing. is awaited.

[Conventional technology]

When looking at conventional chrome stainless steel sheet materials from the viewpoint of two strengths, firstly, martensitic stainless steel is well known as a chrome stainless steel having high strength. For example, JIS G 4305 stipulates seven types of steel as martensitic stainless steel for cold rolled stainless steel sheets. These martensitic stainless steels have carbon content ranging from 0.08% or less (SUS410S) to 0.
60 to 0.75% (SO5440A), compared to ferritic stainless steel at the same -Cr content level.
It contains high C and can be given high strength by hardening treatment or hardening and tempering treatment. For example, this J
In IS G 4305, 0.26-0.40%
In 5US420J2, which contains C and 12.00 to 14.00% Cr, it is possible to obtain a hardness of HRC40 or higher by quenching by rapid cooling from 980 to 1040°C and then tempering by air cooling at 150 to 400°C. and 0.60-0.75% C and 16.00-1
For 5US440A containing 8.00% Cr, 1
After quenching by rapid cooling from 010 to 1070℃, 150℃
It has been shown that the same hardness (HRC 40 or higher) can be obtained by air-cooling tempering at ~400°C.

On the other hand, ferritic stainless steel sheets, which are chromium stainless steels, cannot be expected to undergo much hardening through heat treatment, so one way to increase their strength is to perform cold temper rolling after annealing to increase strength through work hardening. things are being done.

However, the reality is that ferritic stainless steels are not often used in applications that inherently require high strength.

[Problem that the invention seeks to solve]

In martensitic stainless steel sheets, the structure after quenching or quenching and tempering is basically a martensitic structure, as the name suggests.

Although very high strength and hardness are obtained, elongation is very low. Therefore, if quenching or quenching and tempering treatment is performed, subsequent processing becomes difficult. In particular, processing such as press forming is not possible after quenching or quench-tempering. Therefore, when processing is performed, it is performed before quenching or quenching and tempering. In other words, the material manufacturer states that it is in an annealed state, that is, JIS G 4.
As shown in Table 16 of 305, it is usually shipped in a soft state with low strength and hardness, and after being processed into a shape almost similar to the final product at a processing manufacturer, it is quenched or quenched and tempered. be. The surface oxide film (scale) produced by this quenching or quenching and tempering treatment is often undesirable for stainless steel, where surface beauty is important, and as a countermeasure, heat treatment in a vacuum or inert gas atmosphere is recommended. This requires steps such as applying heat treatment and removing scale by polishing or the like after heat treatment. In any case, in order to obtain high strength with martensitic stainless steel sheets, a heat treatment process at the processing manufacturer is essential, which increases the burden on the processing manufacturer, and this increases the cost of the final product. The problem was that I couldn't do it.

On the other hand, when the strength of a ferritic stainless steel sheet is increased by temper rolling, the elongation decreases significantly and the strength-ductility balance deteriorates.

This results in poor workability. The degree of increase in strength due to temper rolling is significantly higher in yield strength than in tensile strength. For this reason, when the rolling rate becomes high, the difference between proof stress and tensile strength becomes smaller, and the yield ratio (= proof stress / tensile strength) becomes close to 1, the plastic working area of the material becomes very narrow, and the yield strength becomes high. This increases the springback and deteriorates the shapeability after press working. Furthermore, the temper-rolled material has very large in-plane anisotropy in strength and elongation, and even light press processing results in poor shape after processing. Furthermore, since the processing strain caused by rolling is larger closer to the surface of the plate, it is inevitable that the strain distribution in the thickness direction of the temper-rolled material will become non-uniform. This results in non-uniform distribution of residual stress in the plate thickness direction, which may cause changes in shape such as warping of the plate after punching or photo-etching, especially in ultra-thin steel plates. This is a big problem in the applications where it is needed. In addition to the above-mentioned problems in terms of material properties, skin-pass rolled materials also have many problems in their manufacturability. First, regarding the control of strength, since temper rolling utilizes work hardening due to cold rolling, the rolling rate is the most important factor determining strength.

therefore.

In order to achieve excellent plate thickness accuracy as a finished product and to stably obtain the target strength level with high accuracy, strict control of the rolling rate, specifically strict control of the initial plate thickness before temper rolling, is extremely important. In addition to this, it is necessary to control the strength level of the material before temper rolling. In addition, in terms of shape control, skin pass rolling and temper rolling, which have a light rolling rate of at most 2 to 3% for the purpose of shape correction, are different from skin pass rolling and temper rolling, which have a light rolling rate of at most 2 to 3%. Since this is essentially cold rolling with a rolling reduction of several tens of percent, it is difficult to obtain a copper strip with excellent shape properties as cold-rolled.

Therefore, for the purpose of shape correction, it may be necessary to heat the material to a temperature range lower than the recovery/recrystallization temperature range at which it does not soften, and to perform stress relief treatment. As described above, temper-rolled materials have many problems in terms of manufacturability.

In addition to the problems caused by the above-mentioned temper rolling, ferritic stainless steel sheets also have the problem of ridging, which can be said to be an essential drawback. Rigging is normal.

It is a type of surface defect that occurs when a cold rolled annealed ferritic stainless steel plate is subjected to processing such as press forming.
Even after cold rolling, ridging, generally called cold rolling ridging, may occur.

This is still a big problem in applications where surface roughness is important.

[Means for solving the problem] The above-mentioned problem is solved by using chrome that has moderately high strength, good ductility and workability that can be processed into a desired shape, and has small anisotropy and does not cause ridging. This problem could be solved if stainless steel materials could be provided in the form of steel plates or steel strips by material manufacturers. In order to solve this problem, the present inventors have continued extensive research on chromium stainless steel from both the chemical composition and manufacturing conditions. As a result, we have properly controlled the steel composition, and as a manufacturing condition, after hot rolling and, in some cases, further hot-rolled sheet annealing, we have developed a method of cooling that is more than plate round with intermediate annealing in the ferrite single phase region. Instead of performing inter-rolling to produce a cold-rolled steel strip with the product thickness, and then final annealing the cold-rolled steel strip at a temperature in the conventional ferrite single phase region, that is, annealing that is applied to a steel plate or steel strip product, If continuous finishing heat treatment is performed under specific conditions consisting of heating to a proper ferrite + austenite dual phase region followed by rapid cooling, a complex state in which a soft ferrite phase and a hard martensite phase are uniformly mixed is obtained. We were able to obtain a remarkable result in that it was possible to obtain a phase structure, and virtually all of the above-mentioned problems could be solved.

Thus, the present invention.

In weight%.

C: 0.15% or less.

Si: 2.0% or less.

Mn: 1.0% or less.

P: 0.040% or less.

S: 0.030% or less.

Ni: 0.60% or less.

Cr: More than 14.0% and less than 20.0%.

N: 0.12% or less.

○: 0.02% or less.

and, in some cases, further contain 0.20% or less of A.
e, B of 0.0050% or less, M of 1.0% or less
o, steel containing one or more types of REV of 0.10% or less and 0.20% or less of Y, with the balance consisting of Fe and unavoidable impurities, and 0.03%≦C+N≦0 A process of producing a steel slab that satisfies the .20% relationship and hot rolling it to produce a hot rolled steel strip.

A process of producing a cold-rolled steel strip of product thickness by two or more cold rollings sandwiching intermediate annealing in the ferrite single-phase region temperature heating;
and.

The obtained cold rolled steel strip is passed through a continuous heat treatment furnace.

After maintaining the temperature in the two-phase region of ferrite + austenite from above the Ac+ point to 100°C for less than 10 minutes.

Average cooling rate 1℃/s from maximum heating temperature to 100℃
Continuous finishing heat treatment step in which finishing heat treatment is performed by cooling at ec or more and 500° C./sec or less.

To provide a method for producing a chromium stainless steel strip having a high ductility and high strength multi-phase structure (m weave substantially consisting of ferrite and martensite) with small in-plane anisotropy and a hardness of IIV 200 or more. It is something.

According to the method of the present invention, not only can substantially all of the above-mentioned problems be solved, but also the strength can be freely and easily adjusted by controlling the heating temperature and cooling rate during AM formation or finishing heat treatment. It provides an advantageous and new manufacturing technology for the industrial production of chromium stainless steel sheets or copper strip materials, and it can be used to manufacture martensitic stainless steel sheets or strips or ferrite stainless steel sheets or strips that have been conventionally shipped on the market. The purpose is to provide the market with a new chromium stainless steel material that has both ductility and strength characteristics that do not exist in other materials, and has less in-plane anisotropy in ductility and strength. 2. According to the method of the present invention, the product that has undergone the final continuous finishing heat treatment process is industrially manufactured in the form of steel strip, and when it is shipped to the market, it is left as a strip (coil). Or it will be shaped into a steel plate.

Conventionally, 2 For example, even in 5US430, which is a representative steel type of ferritic stainless steel, austenite is generated when heated to a temperature in the two-phase region, and this austenite is transformed into martensite by rapid cooling, resulting in a two-phase structure of ferrite + martensite. That itself was known. However, in the production of ferritic stainless steel strips that produce austenite at high temperatures, the heat treatment after cold rolling is only annealing at a temperature in the ferrite single phase region, and high-temperature heat treatment that produces martensite is not recommended. It is common sense to avoid this because it causes deterioration in material quality such as a decrease in ductility, and it has not been considered at all in the actual manufacturing of industrial steel strips.

Therefore, assuming continuous heat treatment as in the present invention after the cold rolling process of chrome stainless steel, and performing finishing heat treatment to heat to the ferrite + austenite dual phase region, the relationship between heating temperature, strength and ductility, and the relationship between ductility and strength. There have been no detailed studies on anisotropy, etc. The present invention provides a completely new manufacturing method that has not been considered as an industrial manufacturing method for high-strength chromium stainless steel strips. As a result, a new chrome stainless steel sheet material with excellent properties not possessed by conventional chrome stainless steel sheets or belts is provided.

[Detailed description of the invention]

Below, the reason for limiting the range of chemical composition values of steel regulated by the present invention and the content of each manufacturing process adopted by the method of the present invention will be specifically explained in detail.

First, the reason for limiting the content range (wt%) of the components of the chromium stainless steel to which the method of the present invention is applied is as follows.

Since C and N are two strong austenite-forming elements and have a large ability to strengthen martensite, they are effective elements for controlling and increasing the strength after continuous finishing heat treatment. Therefore, after continuous finishing heat treatment process, 10%
In order to obtain a multi-phase structure containing the above martensite and sufficient strength of Hv200 or more, the amount (C + N) of at least 0.03% or more is required. However, if the C and N contents are too high, the amount of martensite generated after the continuous finishing heat treatment process will increase, and in some cases it will become 100% martensite, and the hardness of the martensite phase itself will become very high, so high strength will not be achieved. However, the ductility decreases. Therefore, it is necessary to set the (C+N) amount to 70.20% or less, and to satisfy the relationship 0.03%≦C+N 50.20%, and the C amount should be 0.15%.
% or less.

Further, it is difficult to add a large amount of N due to its solubility, and addition of a large amount causes an increase in surface defects, so the amount is set to 0.12% or less.

Si is a ferrite-forming element and has a strong solid solution strengthening ability for both ferrite and martensite phases. Therefore, it is an effective element for controlling the amount of martensite and the strength level. However, adding a large amount leads to a decrease in hot workability and cold workability, so 2.0%
is the upper limit.

Mn and Ni are austenite forming elements.

It is an effective element for controlling the amount of martensite and strength after continuous finishing heat treatment. However, if a large amount is added, the product becomes expensive, which affects the economy, which is one of the characteristics of the steel strip of the present invention. Therefore, the normally allowed limit of M
The upper limits are n: 1.0% and Ni: 0.6%.

If S is too high, it will adversely affect corrosion resistance and hot workability, so it is preferably lower, and the upper limit is 0.030%.

Although P is an element with a large solid solution strengthening ability, addition of a large amount may lead to a decrease in toughness.

Cr is the element that has the most important effect on the corrosion resistance of stainless steel, and in order to obtain sufficient corrosion resistance, 14.0
It should be contained in excess of %.

If the Crl content is high, the amount of austenite-forming elements necessary to generate a martensite phase and obtain high strength increases, and the product becomes expensive, so the upper limit is set at 20.0%.

0 forms oxide-based non-metallic inclusions and lowers the cleanliness of the steel, so a lower value is desirable, and it is set at 0.02% or less.

1M is an effective element for deoxidizing and has the effect of significantly reducing A2-based inclusions that adversely affect press workability. However, if the content exceeds 0.20%, the effect not only becomes saturated, but also brings about adverse effects such as an increase in surface defects, so the upper limit is set at 0.20%.

B is an effective component for improving toughness, but its effect is brought about in a very small amount, and the effect is saturated when it exceeds o, ooso%, so its upper limit is set at o,ooso%.

Mo is an element effective in improving corrosion resistance.

If added in large amounts, the product becomes expensive, so the upper limit is set at 1.0%.

REV and Y are elements effective in improving hot workability. It is also an effective element for improving oxidation resistance. The method of the present invention, which performs continuous finishing heat treatment at high temperatures, is effective in suppressing the generation of oxidized scale and obtaining a good surface texture after descaling. However, these effects
M saturates when it exceeds 0.10% and Y exceeds 0.20%, so the upper limit is 0.10% for REV and 0.10% for Y.
is 0.20%.

Next, the contents of each manufacturing process of the multi-phase steel strip according to the present invention will be explained.

In the method of the present invention, a slab of chromium stainless steel adjusted to one or more steel composition ranges is produced by conventional steel casting techniques, and this slab is produced by conventional hot rolling to produce a hot-rolled steel strip. After hot rolling, it is preferable to perform hot rolled sheet annealing and descaling. Although it is not necessary to carry out hot-rolled sheet annealing, it is possible to soften the hot-rolled steel strip and improve its cold-rollability, and to reduce the residual transformed phase (austenitic phase at high temperatures) in the hot-rolled steel strip. This is desirable for producing a steel strip with a uniform multi-phase structure after cold rolling and continuous finishing heat treatment. This hot-rolled sheet annealing may be either continuous annealing or box annealing, and the descaling step may be carried out by ordinary pickling.
For the hot-rolled sheet annealing process and the descaling process, conventional chrome stainless steel strip manufacturing techniques can be applied to the method of the present invention as they are.

Next, a dual-phase steel strip is manufactured through a cold rolling process and a continuous finishing heat treatment process, and these processes are characteristic of the method of the present invention and will be explained in detail.

"Cold rolling process" In the cold rolling process, a hot rolled steel strip (a hot rolled steel strip after annealing a hot rolled sheet) is cold rolled two or more times with intermediate annealing at a temperature in the ferrite single phase range to form a product sheet. This is the process of rolling to a thick thickness. This intermediate annealing is a multiple set after the continuous finishing heat treatment process! 1
It plays an important role in reducing the in-plane anisotropy of the ductility of the steel strip. This will be explained based on typical test results.

Steels A, B, and C having the chemical composition shown in Table 1 were melted and hot-rolled to a thickness of 3.6 mm under normal conditions, heated at 780°C for 6 hours, and heated in a furnace. After cold annealing, pickling was performed. Tests were conducted using this hot-rolled sheet by changing the cold rolling conditions and finishing heat treatment conditions (the data in Figures 1 and 2 also show the results of this test, the details of which will be described later). ).

Table 2 below is for steel B in Table 1.

(a), a multi-phase m-woven material (hereinafter referred to as ZC
(referred to as R material).

Yama), multi-phase structure material (hereinafter referred to as ICR
material).

(C1, temper-rolled material that has the same strength as ICR material and ZCR material by cold rolling.

The tensile strength (kgf
/mm”) and elongation (χ) in the rolling direction (shi), the value at 45° to the rolling direction (D), and the value at 90° to the rolling direction (T). .

For the 2CR material in (a), the above-mentioned hot-rolled plate was cold-rolled to a thickness of 11, and after intermediate annealing at 800°C for 1 minute and air-cooled, the plate was further cold-rolled to a thickness of 0.3m+n. This cold-rolled plate was heated to 970°C for 1
After soaking for a minute, finishing heat treatment was performed by cooling from that temperature to 100°C at an average cooling rate of 20°C/sec.

In addition, the IRC material (bl) is made by cold rolling the above-mentioned hot-rolled sheet to a thickness of 0.3m+* without intermediate annealing, and after soaking this cold-rolled sheet at a temperature of 970°C for 1 minute. Finishing heat treatment was performed by cooling from temperature to 100°C at an average cooling rate of 20°C and 7 seconds.

(For C1O temper rolled materials, IC'R materials and zC
The same strength as R material with a plate thickness of 0.311! After the annealing, the hot rolled plate was cold rolled to a predetermined thickness, annealed, and then temper rolled at a predetermined rolling rate so as to obtain a state of 1.

As is clear from Table 1 and Table 2, the elongation of the dual-phase Mi woven materials for both ZCR and ICR materials is significantly superior to that of temper-rolled materials with the same hardness and strength levels9. It can be seen that the strength-elongation balance is excellent. In addition, looking at the two-plane anisotropy, in terms of tensile strength, both the 2CR material and the ICR material have a small difference in tensile strength depending on the direction, that is, the in-plane anisotropy of the multiphase & II woven material. The material has the lowest tensile strength and the difference between the tensile strength in the T direction and the highest tensile strength is 1
The in-plane anisotropy is greater than 7 kgf/+am'', and in terms of elongation, the multi-phase structure material with high elongation also has relatively smaller in-plane anisotropy than the temper-rolled material with low elongation.

In particular, it can be seen that the ZCR material has even smaller in-plane anisotropy than the ICR material. In other words, it can be said that intermediate annealing is very important in reducing the in-plane anisotropy of elongation of the multiphase structure material. Therefore, from the results in Table 2, when finishing heat treatment is applied to create a multi-phase structure through hot rolling, hot-rolled sheet annealing, and cold rolling with intermediate annealing, the result is excellent ductility, strength, and ductility. It is clear that a high-strength chromium stainless steel sheet with a multi-phase m weave having a small in-plane anisotropy can be obtained.

As seen in the test results and as shown in the examples below, the in-plane anisotropy of the elongation of the multiphase structure material that remains even after the continuous finishing heat treatment according to the present invention is The process can be made smaller by performing two or more cold rollings with intermediate annealing in between. Therefore, in producing a dual-phase steel strip with small ductile in-plane anisotropy.

The plate thickness is reduced to the product plate thickness by cold rolling two or more times,
It is important in the method of the present invention to perform intermediate annealing during that time. The heating temperature for this intermediate annealing is the ferrite single phase region temperature, that is, Ac, the intended temperature. In addition, the cold rolling ratios of cold rolling before and after intermediate annealing are each at least 30.
It is better to set it to % or more.

"Continuous finishing heat treatment process" The cold-rolled steel strip with the product thickness obtained in the cold rolling process is then passed through a continuous heat treatment furnace, and the two-phase region temperature of ferrite decaustenite is higher than the Ac+ point and lower than 1000℃. After holding for 10 minutes or less, a continuous finishing heat treatment is performed in which the material is cooled from the maximum heating temperature to 100°C at an average cooling rate of 1°C/sec to 500°C/sec.

This continuous finishing heat treatment step is the most characteristic step of the method of the present invention, and the conditions for this continuous finishing heat treatment have an important meaning in the present invention, as will be shown in Examples below. An overview of the reasons for regulating the heating conditions and cooling conditions in this continuous finishing heat treatment step is as follows.

It is an absolute condition that the heating temperature during the continuous finishing heat treatment is in the ferrite + austenite dual phase region temperature. When carrying out the method of the present invention, when heating from a low temperature in a continuous heat treatment furnace, near the temperature at which austenite begins to form (that is, the temperature at the Ac+ point), the amount of austenite fluctuates greatly in response to temperature changes, and stable hardness cannot be obtained after rapid cooling. There may be no.

However, in the steel composition range targeted by the present invention,
It has been found that when heated to a high temperature range of 100° C. or more from the point Ac, such fluctuations in hardness do not substantially stop. Therefore, the heating temperature during the continuous finishing heat treatment is preferably set to Act point +100°C or higher.

More specifically, 900°C or higher, more preferably 950°C
It is better to keep the temperature above ℃. On the other hand, regarding the upper limit of the heating temperature, if the temperature is too high, the increase in strength not only becomes saturated, but also decreases in some cases.

Further, since it is disadvantageous in terms of manufacturing cost, it is preferable to set the upper limit to 1100°C.

The metallurgical significance of heating in the ferrite + austenite two-phase region during continuous finishing heat treatment in the method of the present invention is as follows: ■CrCr
Three points can be mentioned: carbide-nitride dissolution, (1) generation of austenite phase, and (2) concentration of C and N in the generated austenite.

In the case of the chromium stainless steel strip targeted by the method of the present invention, all of these phenomena reach an almost equilibrium state within a short time. The heating time may be short, approximately 10 minutes or less. This short heating time is very advantageous in terms of production efficiency and manufacturing cost in the actual operation of the method of the present invention. Depending on the holding time, it is possible to generate austenite necessary for the amount of martensite generated by subsequent cooling to be 20% by volume or more.

The cooling rate during finishing heat treatment is set at 1°C/1°C in order to obtain a multi-phase structure of martensitic phase and soft ferrite phase.
It is necessary to set the cooling rate to sec or more, but the cooling rate is 500℃/
It is virtually difficult to obtain cooling rates in excess of sec.

Therefore, in the present invention, cooling from two-phase temperature range heating is performed at a cooling rate of 1 to b. This cooling rate is an average cooling rate from the maximum heating temperature to 100°C, but this cooling rate does not necessarily need to be adopted in the cooling process after austenite has been transformed into martensite. This cooling rate and cooling end point temperature are sufficient for the austenite produced at high temperature under the above-mentioned heating conditions to transform into martensite. As a cooling method, a forced cooling method in which a gas and/or liquid cooling medium is sprayed onto the belt, a roll cooling method using water-cooled rolls, etc. can be applied. Continuous heating and cooling under such conditions can be carried out using a continuous heat treatment furnace having a heating soaking zone and a quenching zone between the coil unwinding machine and the winding machine.

FIG. 1 shows hot-rolled sheets (hot-rolled sheets after hot-rolled sheet annealing and pickling) manufactured by the method already described for each steel in Table 1 above, which are cold-rolled to a thickness of 800 mm. ℃×
After performing intermediate annealing by heating and air cooling for 1 minute, the cold rolled plate is further cold rolled to a thickness of 0.3 mm, and this cold rolled plate is heated at each temperature between 800 and 1150°C for 1 minute. Amount of martensite (volume %) in finished heat-treated material obtained when finishing heat treatment is performed by soaking and then cooling from that temperature to 100 °C at an average cooling rate of 20 °C/sec
and hard HV).

This is shown in relation to the heating temperature during finishing heat treatment (
A, B, and C in the figure represent each steel in Table 1).

As is clear from FIG. 1, when the heating temperature exceeds 800°C and enters the ferrite + austenite dual phase region, martensite appears after the second finishing heat treatment.

The amount of martensite increases rapidly as the heating temperature rises, but when the temperature exceeds 900 to 950°C, the degree of increase decreases and it tends to gradually become saturated.

The behavior of hardness also shows a similar tendency in response to changes in the amount of martensite, and the higher the amount of martensite, the higher the hardness. It has important significance. In other words, a certain degree of temperature fluctuation is unavoidable in a continuous heat treatment line.
In particular, there are variations in the lengthwise direction of the steel strip, and differences in heat treatment temperature due to differences in threading opportunities even if the target temperature is the same.

In actual line operation, it is necessary to allow for fluctuations of about ±20°C with respect to the target temperature. Figure 1 shows that if the cooling rate is kept almost constant and a heat treatment temperature range with small hardness fluctuations is adopted, even if there is some temperature fluctuation in the continuous heat treatment line, the hardness, that is, the strength, of the copper strip will be small. This shows that it can be manufactured. If one strength level is controlled by component control as described above, the target strength can be stably obtained, and strength fluctuations over the entire length of the steel strip are small, and strength differences between strips are small. Strong materials can be manufactured easily and inexpensively using existing continuous heat treatment lines.

Figure 2 shows the correlation between hardness and elongation (weighted average value in three directions) of several multiphase materials with different amounts of martensite made within the range of steel composition and manufacturing conditions regulated by the present invention. This is shown in comparison with the II quality rolled material. Note that the production of the multi-phase structure material is the same as that explained in FIG. 1, and the heating temperature in the finishing heat treatment is 900° C. or higher.

Further, the temper-rolled material is obtained by annealing after cold rolling, and then changing the hardness by changing the temper rolling rate indicated by the subscript in the figure.

As is clear from FIG. 2, the elongation of the temper-rolled material sharply decreases as the hardness increases as the temper rolling rate increases.

On the other hand, even if the hardness of the multiphase structure material increases, the elongation decreases slowly. In particular, the elongation of the multiphase structure material is superior to that of the m-rolled material in the region of high hardness, specifically in the Hv 2
This becomes noticeable in the region of 00 or more. That is, multiphase &l
The increase in ductility achieved by using an I-woven material becomes even more remarkable in the region of Hv200 or more, and for this purpose, as can be seen from the above-mentioned Figure 1, it is necessary to increase the ductility at a martensite content of about 10% by volume or more. be. In this way, the hardness is Hv
The multi-phase structure material obtained by the present invention method is characterized by the ability to achieve high ductility of 200% or higher, which cannot be achieved with temper-rolled materials. The steel strip also has properties such as press formability that cannot be obtained by temper rolling.

Figure 3 is a photo of the metallographic structure when w4B in Table 1 is manufactured using the method (at in Table 2).The whitish area in the photo is ferrite, and the darker gray area is martensite. As you can see from this photo,
This material has a multi-phase structure in which fine ferrite and martensite are uniformly mixed.

As explained above, the high ductility and high strength belt material with low strength and ductility anisotropy was obtained by hot rolling.
After hot-rolled plate annealing and two or more cold rollings with intermediate annealing in between, finishing heat treatment involves heating to a two-phase region of ferrite and ten austenite and rapidly cooling, resulting in fine ferrite and martensite that is transformed from austenite by rapid cooling. This was achieved by creating a multi-phase structure with a uniform mixture of In other words, 5 hard martensite provides strength (hardness), soft ferrite provides ductility, and the in-plane anisotropy of strength and ductility is achieved by finely and uniformly mixing both phases. It could have been made smaller. Note that a trace amount of retained austenite may be detected in X-ray examination of the structure after finishing heat treatment.

Below are nine examples in which the method of the present invention was implemented.

The characteristics of a multi-phase steel strip obtained by the method of the present invention will be specifically shown in comparison with a comparative example.

Example Slabs were manufactured by melting steel having the chemical components shown in Table 3. After hot rolling to a plate thickness of 3.61,
The hot-rolled plate was annealed by heating at 80°C for 6 hours and cooling in a furnace, and after pickling, it was cold-rolled under the cold-rolling conditions shown in Table 4 to obtain a plate with a thickness of 0.3.
A cold-rolled steel strip having a thickness of 1.5 mm was prepared and subjected to continuous finish heat treatment in a continuous heat treatment furnace under the finish heat treatment conditions shown in Table 4. The soaking time for intermediate annealing in the cold rolling process is 1 minute, and the soaking time for the continuous finishing heat treatment process is also 1 minute. The material properties of the steel strip after finishing heat treatment are also listed in Table 4.

As is clear from Table 4, according to the method of the present invention, both multiphase #A had high tensile strength, hardness, and good elongation.
It can be seen that a woven steel strip was obtained. Furthermore, it is clear that the band prepared by the method of the present invention has small anisotropy in 0.2% proof stress, tensile strength, and elongation, and no ridging is observed in the tensile test piece after fracture.

On the other hand, in Comparative Example Nll, the manufacturing conditions are within the range specified by the present invention, but the C and N@ of the steel are lower than the conditions for the present invention steel (C+N) of 20.03%, (C++N).
-0,021% 18Cr of sea bream (Arashi 8 steel in Table 3)
Therefore, austenite does not form even at high temperatures, so it remains a ferritic Yuwa steel even after continuous finishing heat treatment. Therefore, the strength and hardness are low.

In Comparative Example Light 2, the manufacturing conditions are still within the scope of the present invention, but the Cl in g is higher than the amount of C defined in the present invention (C50,15
%), the steel has a C-0.155% (tit of Magnetic 9 in Table 3), and the amount of (C-N) also exceeds the 0.20% stipulated in the present invention, so continuous The amount of martensite after finishing heat treatment is 100%.

In Comparative Example 3, which has high strength but very low elongation, the heating temperature in the continuous finish heat treatment is low, and at this heating temperature, the 1IN12 steel does not enter the ferrite + austenite dual phase region, so after the finish heat treatment, the □ The metal structure is a single-phase ferrite weave with no martensite, and has high elongation but low strength and hardness.

Comparative Example No. 4 was subjected to finishing heat treatment in a box furnace.

Since the cooling is also by furnace cooling, the cooling rate is 0.03℃/se
c, which is very low, so no martensite is generated after heat treatment, and like Comparative Example No. 3, the elongation is high, but the strength and hardness are low.

Comparative Example No. 5 is a rolled material made of tm', which has significantly lower elongation than that of the present invention, and has a tensile strength of 0.
, 2% proof stress ratio, that is, the yield ratio is high, and 0,2
It is clear that the anisotropy of % proof stress, tensile strength, and elongation is large, and therefore the processability and shape after processing are inferior to the toning belt obtained by the method of the present invention.

Comparative Example 6 has high strength and excellent elongation because no intermediate annealing was performed during cold rolling before continuous finishing heat treatment, but the in-plane anisotropy of elongation was lower than that of the present invention example that underwent intermediate annealing. It's bigger compared to the others.

In addition, for the comparative examples Nll, 3.4 and 5, the occurrence of ridging was observed in the tensile test specimens after rupture, whereas the multi-phase &IIm steel strip of the present invention example 9 showed no occurrence of ridging. It can be seen that processing such as press molding can be performed satisfactorily.

As described above, according to the method of the present invention, a dual-phase steel strip having both high ductility and high strength, small in-plane anisotropy of strength and ductility, low proof stress, and low yield ratio is provided. In the field of chrome stainless steel sheets, there has never been an example of such a high-strength material with Hv200 or higher combined with good workability being shipped to the market in the form of a steel sheet or copper strip. Therefore, the present invention provides a new material steel sheet or belt for the conventional chrome stainless steel sheet field. The material according to the present invention is particularly useful as a high-strength material that requires workability into electronic parts, precision mechanical parts, etc. It is useful and much work can be done in this field.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the heating temperature of the finishing heat treatment, the amount of martensite, and the hardness according to the present invention. FIG. 2 is a diagram showing the correlation between hardness and elongation for finish heat-treated materials and temper-rolled materials according to the present invention. FIG. 3 is a micrograph showing the metallographic structure of a chromium stainless steel strip subjected to continuous finishing heat treatment according to the present invention.

Claims (6)

[Claims] (1) In weight%, C: 0.15% or less, Si: 2.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0 Steel containing: .60% or less, Cr: more than 14.0% and 20.0% or less, N: 0.12% or less, O: 0.02% or less, with the balance consisting of Fe and inevitable impurities. A process of manufacturing a steel slab that satisfies the relationship of 0.03%≦C+N≦0.20%, and hot rolling it to produce a hot rolled steel strip, intermediate of ferrite single phase region temperature heating. A process of producing a cold-rolled steel strip of product thickness by cold rolling two or more times with annealing in between;
Then, the obtained cold rolled steel strip was passed through a continuous heat treatment furnace to obtain Ac_1
After maintaining the ferrite + austenite two-phase region temperature of 1100°C or higher for less than 10 minutes, the average cooling rate from the maximum heating temperature to 100°C is 1°C/sec or more and 500°C.
A method for producing a highly ductile, high-strength, multi-phase structure chromium stainless steel strip with a hardness of HV200 or more and a small in-plane anisotropy, the method comprising: a continuous finishing heat treatment process in which finishing heat treatment is performed by cooling at a temperature of 0.degree. C./sec or less. (2) The heating temperature in the continuous finishing heat treatment process is Ac_1
The manufacturing method according to claim 1, wherein the temperature is +100°C or higher and 1100°C or lower. (3) The heating temperature in the continuous finishing heat treatment process is 900℃
The manufacturing method according to claim 1, wherein the temperature is above 1100°C. (4) In weight%, C: 0.15% or less, Si: 2.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0 .60% or less, Cr: over 14.0% and 20.0% or less, N: 0.12% or less, O: 0.02% or less, and Al of 0.20% or less, 0.0050% or less B, 1.0% or less Mo, 0.10% or less REM, 0
．． A steel slab containing 20% or less of one or more types of Y, with the remainder consisting of Fe and unavoidable impurities, and which satisfies the relationship 0.03%≦C+N≦0.20%. A process of producing a hot-rolled steel strip by hot-rolling the product, and a process of producing a cold-rolled steel strip of product thickness by cold rolling two or more times with intermediate annealing in the ferrite single-phase range temperature heating. ,
Then, the obtained cold rolled steel strip was passed through a continuous heat treatment furnace to obtain Ac_1
After maintaining the ferrite + austenite two-phase region temperature of 1100°C or higher for less than 10 minutes, the average cooling rate from the maximum heating temperature to 100°C is 1°C/sec or more and 500°C.
A method for producing a high-ductility, high-strength, multi-phase structure chromium stainless steel strip having a hardness of HV200 or more and a small in-plane anisotropy, the method comprising: a continuous finishing heat treatment step of performing a finishing heat treatment by cooling at a temperature of not more than 0.degree. C./sec. (5) The heating temperature in the continuous finishing heat treatment process is Ac_1
The manufacturing method according to claim 4, wherein the temperature is +100°C or higher and 1100°C or lower. (6) The heating temperature in the continuous finishing heat treatment process is 900℃
The manufacturing method according to claim 4, wherein the temperature is above 1100°C.